The limitation on seller concessions is set by the type of financing you choose more than anything else. Conforming financing will allow 3% of the sales price as seller concessions. FHA allows 6% of the purchase price to be paid by the seller for concessions.
Check with your lender. Get pre-approved and then have your lender communicate with your realtor the specifics about seller concessions so the contract can be correctly written.
What are the concessions to cover? If he is asking you that to cover the closing costs of the loan, I would encourage you to shop around for loan officers and programs. If you are doing an FHA loan most places can offer those alot cheaper than 3000. There also might be pre-paids included with that. In my state, my realtor said HUD will not pay closing cost's, but I would definately ask your realtor for advice. Good Luck!!